FROM PLIOCENE CLAY BEDS IN COSTA RICA. 377 



To form a faint idea of the number of cases in which generic names are repeated 

 in various orders, let the reader refer to the general index of Agassiz's Nomenclator 

 Zoologicus; and then remember that that only tells us the state of things thirty 

 years ago, when names were probably not half so numerous as they are now. 



CARDITA, Brug. 

 C. CoNEADiANA, Gabb, n. s., PI. 47, fig. T9. 



Shell minute, oblique; beaks very prominent and pointed strongly in advance; 

 posterior cardinal line sloping rapidly ; posterior end abruptly truncated ; anterior 

 end and base rounded ; anterior end somewhat excavated under the beaks, the 

 lunule large and prominent. Surface marked by about eighteen large, curved, 

 beaded ribs. 



Length 1| mm. 



LITHOPHAGA, Bolt. 

 L. INCURVA, n. s., PI. 4T, fig. 80. 



Shell small, very convex, thicker than wide, arcuated ; beaks anterior, very 

 prominent, and distinctly incurved ; anterior end produced below, basal margin 

 concave; surface marked only by heavy, irregular lines of growth. 



Length 0.9 inch; width 0.35 inch; diameter measured through the greatest 

 convexity of the two valves 0.5 inch. 



But a single specimen was found but which, fortunately, gave both valves sepa- 

 rated in a beautifully perfect condition. 



CRENELLA, Brown. 

 Sub-gen. MODIOLARIA, Beck. 

 C. (M.) TRANSLUCiDA, Gabb, n. s., PL 41, fig. 81. 



Shell minute, very thin, translucent ; elongated ; beaks large and overhanging 

 the anterior end; posterior cardinal margin nearly horizontal; posterior end 

 broadly rounded and sloping outward ; anterior end not prominent ; base divided 

 into three parts, each of which has a broadly convex outline. The posterior third 

 of the shell is marked by numerous fine, radiating, and slightly curved ribs. The 

 middle is smooth, and the anterior end is marked by similar ribs. 



Length of largest specimen is 4 mm. 



This shell is very similar to C. (M.) discors, Linn., in form and ornament, but 

 the ribs are finer and more numerous, and the shell is altogether more oblique, 

 more produced in the posterior basal region. 



76 



